Study On Radio Frequency Drying Protocol Of Macadamia Nuts

Macadamia nuts (Macadamia tetraphylla) are native to Australia and growncommercially in Australia, Hawaii, South Africa, and South America. The world macadamiakernel production was estimated to be100,000metric tons in2010/2011according to thereport of International Nut Council (INC). Studies on macadamia nuts were developed late inChina. From1970s, macadamia nuts were introduced and planted in tropic and subtropicalzones, such as Guangdong, Guangxi, Yunnan and Sichuan provinces. After more than thirtyyears of developments, about6000hm2macadamia nuts were planted in China, with the mainplanting areas located in Guangxi and Yunnan. Macadamia nuts have thick and hard shells,which result in a very long drying cycle, often more than one month to complete the industrialdrying. The long process occupies enormous areas, with excessive handling of the in-processmaterial and considerable costs involved. It is desirable to develop advanced heatingtechnologies to speed up the conventional drying process. Radio frequency (RF) energy hasthe potential to be one of the alternatives since RF heating provides fast and uniform dryingwith acceptable product quality. To develop the optimal hot air assisted RF (RFHA) dryingprotocol of macadamia nuts, experiments were conducted systematically.In this study, the following experiments were conducted successively on macadamia nuts:determining dielectric properties, developing adsorption and desorption isotherms, andstudying RFHA drying of macadamia nuts and RFHA heating uniformity. The detailedresearch contents were completed as follows:①Dielectric properties of the macadamia nutkernels were measured between10and1800MHz using an open-ended coaxial-line probetechnique at temperatures between25and100°C and moisture contents between3%and32%on a dry basis (d. b.).②Adsorption and desorption isotherms of macadamia nut kernelsand shells at room temperature (25℃) were determined. Non-linear regression tool of thesoftware SPSS for Windows (Version16.0, SPSS, Inc.) was used to evaluate the fitness of thesix commonly used models based on the experimental data, and to determine the best modelsand their parameters.③A pilot-scale27.12MHz,6kW RF system was used. Experimentscombined with three temperatures and three electrode gaps were done to select optimal hot airtemperature and electrode gap. Comparisons were made between RFHA drying of nuts andhot air (HA) drying. Six drying kinetic models were fitted with the data of HA and RFHA drying of nuts, using nonlinear regression tool of the software SPSS. Peroxide value and freefatty acid were analyzed of nuts obtained during drying.④Experiments were conducted todetermine the heating and drying uniformity of macadamia nuts. The material was pre-driedmacadamia nuts with averaged moisture content of0.1056kg water·kg dry solid-1andpolyurethane foams. Stacked polyurethane foam sheets were located at different positionshorizontally or vertically and different height from the ground electrode in the RF cavity withor without hot air to investigate the heating uniformity and electric field distribution of thesystem. Comparrisons were made of temperatures measured by fibre optic sensors andinfrared imaging, contour plot and uniformity index derived experimentally for the RF unit.Heating and drying uniformity was conducted with nuts in containers or four layers placed atdifferent positions in the RF cavity with or without hot air under move or static conditions.The main results were described as follows.(1) Both dielectric constant and loss factor of the kernels decreased sharply withincreasing frequency over the RF range (10～300MHz), but gradually over the measuredMW range (300～1800MHz), which were largely enhanced by increasing moisture contentand temperature. Penetration depth decreased with increasing frequency, moisture content,and temperature. Based on this study, uniform drying of macadamia nut kernels in thicklayers could be effectively developed using RF energy.(2) For macadamia nut kernels, the obtained desorption isotherm was of type Ⅱ andadsorption of type Ⅲ according to IUPAC's classification. GAB model was the best fitted fordesorption isotherm and that for adsorption was Henderson model. The parameters (A, B, C)of GAB model were8.2439,0.4815, and1.3545, respectively, for desorption isotherm. Theparameters (A, B) of Henderson model were0.3006and0.8682for adsorption isotherm. Formacadamia nut shells, the obtained desorption and adsorption isotherms were of type Ⅰ,according to IUPAC's classification. The hysteresis loop was of type H3. GAB model was thebest fitted for desorption and adsorption isotherms. The parameters (A, B, C) of GAB modelfor desorption isotherm were9.693,0.605and8.378respectively, and those for adsorptionisotherm were9.695,0.635and3.268.(3) Gap of electrodes played more important role in the heating rate and equilibriumtemperature of samples than hot air in RFHA drying. The optimal combination of gap and hotair temperature was15.5cm and50°C. The drying curves showed an exponential decay andrequired750min and360min to achieve the final moisture content of0.030kg water·kg drysolid-1in whole nuts and of0.015kg water·kg dry solid-1in kernels for hot air and RF heating,respectively. The drying kinetics of the nuts could be well described by Page Model for hotair drying but by Logarithmic model for RF drying. Peroxide value and free fatty acid increased with the drying time both for hot air and RF drying but remained within acceptablerange required by the nut industry. The RF treatment holds potential to provide rapid, uniformand environmentally friendly drying technology for the nut industry.(4) Experiments with foam showed that electric field was not uniformly distributed in RFcavity. There existed corner or edge heating in RF cavity, in which the electric field wasstrong at the corner or the edge of the cavity. When foam stacks was placed at the sameposition horizontally or vertically in RF cavity, it showed that temperatures were higher in thecenter than that all around. There appeared center heating pattern of the foam stacks under thestudied conditions. Hot air was helpful to facilitate the RF heating uniformity of stackedpolyurethane foam cubes. Experiments with nuts showed that edge heating also existed in RFcavity. Under the experimental conditions, moving did not brought noticeable improvementon the heating and drying uniformity. There appeared center heating pattern of RFHA heatingof nuts. Hot air was vital factor for uniform heating and drying of nuts. The determinedRFHA heating and drying conditions were useful for further detailed drying studies of nuts.